[NSRCA-discussion] NiMH Question

[John Ferrell]

I think this procedure is essential with NiMh packs. You can do OK with individual cells (like digital camera applications) using a charger that shuts down with peak detection. When you charge a set of NiMh's in series with a peak detector it stops with the first cell peaking. It doesn't hurt anything, you just don't get a full charge on the pack. 

It is possible to build a pack charger that charges each cell individually but it gets cumbersome. Especially when you consider that an occasional 16 hour charge will solve the problem.

I find the same procedure also works well for NiCad's. If you reset the peak charger and restart it you will cook the  batteries and suffer short battery life. NiCad's normally fail shorted and you have enough power to land. NiMh's normally fail open and you will need another airplane to go with your new battery.

That is also why I use NiMh in the transmitter and NiCads in the flight packs.

John Ferrell    
http://DixieNC.US

  ----- Original Message ----- 
  From: Bob Richards 
  To: NSRCA Mailing List 
  Sent: Friday, February 24, 2006 6:31 AM
  Subject: Re: [NSRCA-discussion] NiMH Question


  The problem with some of the cyclers is that they do a fast charge, then discharge, then fast charge, etc. without ever "equalizing" the cells.  The problem is that one or more cells in the pack can become depressed more than others, and without a good equalization charge they will never be topped off fully since the cells that ARE fully charged will trip the peak detection when they come up.

  I suggest doing a C/10 charge for 16 hours, then cycle the pack and see what happens.

  Bob R.


  Michael Cohen <precisionaero at comcast.net> wrote:
    I am cycling my batteries and getting ready for the flying season.  On my initial 5 cycle run at my NiMH batteries, ! both my 1650 mAh packs cycled very low.  One pack that I have not used since end of November only cycled up to 850 mAh and the other pack I have not used since end of September only cycled up to 1000 mAh.  My charger is set to 5 cycles, charge and discharge are set to .65 amps with a cutoff voltage at 1.1 Volts per cell.  I am now trying 1.00 amp charge/discharge with a 1.0 volt per cell cutoff and it is looking better, but I have some questions:

    1)  Did I damage the batteries by letting them self-discharge too low?  I know NiMH batteries have a higher self discharge rate than NiCD batteries, but I am not aware letting the NiMH batteries run down could cause damage.

    2)  Is this a normal behavior?  Maybe NiMH just need a little more juice at the beginning of the season.

    3)  Is the cold garage a factor?

    Thoughts?

    Mike Cohen
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